1. Field of the Invention
This invention relates to centralizers for use in the construction of oil and gas wells.
2. Description of the Related Art
During the construction of oil and gas wells a borehole is drilled into the ground. A string of casing is then lowered down the borehole. The annular space between the casing and the borehole is then filled with cement to set the string of casing in place.
An extension of the borehole may be drilled and a second string of casing or a liner is then lowered through the first casing string and into the extension of the borehole. It is then set in place.
It is important to ensure that the casing or liner is held centrally in the borehole during cementation and it is usual to provide the casing with a plurality of centralizers which act between the casing and the borehole.
It is also known to use centralizers to centre one tubular within another.
One known type of centralizer is described in co-pending PCT Application Number PCT/GB97/02249 which centralizer comprises a pair of annular bands for encircling a tubular to be centralised. The annular bands are spaced apart by a plurality of ribs inclined to the longitudinal axis of the centraliser.
One problem with such centralizers is that as the casing or liner is lowered into a previously set casing, or into open hole, the friction created between the centralizer and casing/open hole limits the distance which the casing or liner can be pushed from the surface. This problem is particularly acute in wells which have long, near horizontal sections.
PCT Publication Number WO 95/21986 discloses a drill casing installation device with external cylindrical rollers. However, the cylindrical rollers tend to jam in certain conditions.
PCT Publication Number WO 96/34173 discloses a drill string fitting which is both rotational radially and comprises rollers for axial movement. However, the fitting tends to disintegrate if it becomes necessary to push or pull them through a restriction.
According to the present invention there is provided a centralizer which comprises a pair of annular bands which are spaced apart by the plurality of hollow members characterised in that said hollow members are each provided with at least one window having a roller freely rotatably mounted therein.
Other features of the invention are set out in the Claims.